Aliphatic polyesters, such as polyglycolic acid and polylactic acid, can be decomposed by microorganisms or enzymes present in nature, such as soil or sea water, so that they are noted as biodegradable polymer materials giving little load to the environment.
Among the aliphatic polyesters, polyglycolic acid is excellent in gas-barrier properties, such as oxygen gas-barrier property, carbon dioxide gas-barrier property and water vapor-barrier property and also excellent in heat resistance and mechanical properties, and therefore the development of new use thereof is under way singly or in a composite state together with another resin material in the fields of packaging materials, etc.
An aliphatic polyester can be synthesized by dehydro-polycondensation of an α-hydroxycarboxylic acid, such as glycolic acid or lactic acid, while it is difficult to produce a high-molecular weight aliphatic polyester through this process. In contrast thereto, in order to effectively synthesize an aliphatic polyester of a high molecular weight, there has been adopted a process of synthesizing a bimolecular cyclic ester of an α-hydroxycarboxylic acid and subjecting the cyclic ester to ring-opening polymerization. For example, by ring-opening polymerization of glycolide that is a bimolecular cyclic ester of glycolic acid, polyglycolic acid is obtained. By ring-opening polymerization of lactide that is a bimolecular cyclic ester of lactic acid, polylactic acid is obtained.
The polymerization process for producing an aliphatic polyester by ring-opening polymerization of a cyclic ester is generally performed in the form of melt-polymerization where all the steps are operated above the melting point of the product aliphatic polyester, and the present inventors, et. al., have proposed a process wherein a latter half step of the ring-opening polymerization of cyclic ester is performed in a reactor of relatively thin tubes to effect solid-phase polymerization (Patent Document 1 listed below). This process is, however, a batch-wise polymerization process using relatively thin tubes, and a process allowing easier mass production is desired. Further, the polymerizate recovered in the form of solid lumps requires a granulating step, such as pulverization.
There has been proposed a process of condensation-polymerizing lactic acid in a twin-screw stirring apparatus including a former half of melt polymerization and a latter half of solid polymerization, so as to enhance the efficiency of removing generated water and recover the product lactic acid polymer in a pulverized form (Patent Document 2 below). As already mentioned, however, it is difficult to obtain a high-molecular weight aliphatic polyester by poly-condensation of an α-hydroxy-carboxylic acid, such as lactic acid, like this process. Further, this process is essentially a batch-wise process and cannot be regarded as an entirely efficient process for producing an aliphatic polyester.
In contrast thereto, processes of ring-opening polymerization of a cyclic ester in a twin-screw extruder suitable for a high-viscosity polymerizate (Patent Document 3 below) and in a tubular reactor (Patent Document 4 below), have been proposed as continuous processes for producing aliphatic polyesters. However, while an aliphatic polyester is excellent in biodegradability on one hand, the thermal degradability and discoloration under heating thereof cannot be disregarded, so that it is difficult to obtain an aliphatic polyester of a high-molecular weight and with little discoloration.    Patent Document 1: WO 03/006526A1    Patent Document 2: JP 11-279267A    Patent Document 3: JP7-126358A    Patent Document 4: JP10-60101A